1. Field of the Invention
The present invention relates to a backward closed loop power control apparatus for a mobile communication system and a method using the same, and in particular to a backward closed loop power control apparatus for a mobile communication system and a method using the same which are capable of dynamically controlling a backward closed loop power control based on a propagation environment of a forward link and a backward link for a mobile station of a mobile communication system using a code division multiple access(CDMA).
2. Description of the Background Art
Generally, in a CDMA mobile communication closed loop power control design, it is preferred to maximize the number of users capable of communicating each other in a certain bandwidth. In this case, it is possible to maximize the capacity of the system by enabling a transmission power of each mobile station to a base station using a minimum signal-to-noise ratio.
At this time, when the transmission power of the mobile station received into the base station is too low, a bit error ratio is increased. In this case, it is impossible to obtain a desired communication quality. If it is too high, the communication quality of the mobile station is good. However, an interference with respect to another mobile station using the same channel is increased, so that the communication quality is decreased without decreasing the number of users.
In addition, in the mobile communication system, a full duplex wireless channel is used, in which channel a frequency bandwidth used for a signal transmission from the base station to the mobile station and a frequency bandwidth used for a signal transmission from the mobile station to the base station are different. The above-described frequency bandwidth division implements the use of the mobile station transmitter and receiver at the same time without any feedback or interference of the mobile station transmission signal with respect to the mobile station receiver. In addition, the above-described operation greatly affects the power control operation.
In the case of a 45 MHz frequency bandwidth division between the forward link and the backward link, it exceeds a coherence bandwidth of the channel, the loss in a multiple path of the forward link measured in the mobile station is different from the loss in the multiple path of the backward link. In this case, it is assumed that the above-described losses are same since it is impossible to measure the loss of the multiple path of the backward link in the base station.
In the above-described multiple path loss measuring method, an accurate average value with respect to the transmission power is provided. At this time, in order to prevent a Rayleigh fading phenomenon having the different characteristics between both paths, a certain correction method is needed. Namely, the mobile station estimates the signal power from the base station in order to compensate an independent Rayleigh fading phenomenon of the backward link and the forward link.
In other words, the signal-to-noise of the signal received from each mobile station in a demodulator of the base station is measured, and the thusly measured signal-to-noise ratio is compared with a previously set reference signal-to-noise ratio, and a transmission power control instruction is transmitted to the mobile station through the forward channel based on a result of the comparison.
The transmission power control instruction is used to determine the transmission power of the mobile station together with the transmission power estimation value of the open loop method of the mobile station, and the transmission power of the mobile station is increased or decreased by the unit of 1 db in accordance with the instruction transmitted at every 1.25 ms, and it is possible to trace the Rayleigh fading of the backward link.
Here, it is important to decrease the time required for the determination step of the transmission power control instruction and the transmission step of the instruction so that the channel environment is not changed until a control bit is received in the mobile station, and an actual control operation is performed.
In the system controller of the switch station of the mobile communication system, the signal-to-noise ratio is provided to the controller of the base station based on the error ratio of each mobile station, and the signal-to-noise ratio is transferred to the channel controller and is used for a up or down of the transmission power of each mobile station.
The above-described operation is called as a backward closed loop power control method.
The power control operation of the conventional CDMA mobile communication system is performed when a call is connected between the mobile station and the base station. The above-described power control is classified into a forward power control in which a communication frame from the base station and the mobile station is maintained at a certain degree, and a backward power control in which a communication frame from the mobile station to the base station is maintained at a certain degree.
The backward closed loop power control method for a conventional mobile communication system in the above-described power controls will be explained with reference to FIG. 1.
First, the base station receives a communication frame transmitted from the mobile station in a communication frame receiving step S1, and a signal-to-noise ratio is analyzed at every 1.25 ms with respect to the communication frame received in the step SI in a signal-to-noise ratio analyzing step S2.
In a signal-to-noise ratio analyzing step S3, the signal-to-noise ratio analyzed in the step S2 is compared with a previously set reference signal-to-noise ratio. As a result of the comparison of the signal-to-noise ratio comparison step S3, if the signal-to-noise ratio analyzed in the step S2 is smaller than the reference signal-to-noise ratio, a transmission power up instruction is transmitted to the mobile station in a transmission power up instruction transmission step S4, and in a transmission power up step S5, the transmission power of the mobile station is manually increased by 1 dB based on the transmission power up instruction of the step S4.
As a result of the comparison of the signal-to-noise ratio comparison step S3, the signal-to-noise ratio analyzed in the step S2 is larger than the previously set reference signal-to-noise ratio, in a transmission power down instruction transmission step S6, a transmission power down instruction is transmitted to the mobile station, and in a transmission power down step S7, the transmission power of the mobile station is manually decreased by 1 dB based on a transmission power down instruction of the step S6.
In the above-described conventional backward closed loop power control method for a mobile communication system, the transmission power of the mobile station is increased or decreased by the fixed unit of 1 dB in accordance with the transmission power up or down instruction transmitted from the base station.
Therefore, in the backward closed loop power control method for a mobile communication system, in the case that the base station is installed at a bad environment location, since the power amount increased in the mobile station is fixed, the communication quality of the mobile station is decreased, and in the case that the mobile station is installed at a portion in which the propagation environment is bad, since the down amount of the transmission power of the mobile station is constantly fixed, a propagation interference may occur in another mobile station.
Accordingly, it is an object of the present invention to provide a backward closed loop power control apparatus for a mobile communication system and a method using the same which are capable of implementing an economic use of a power of a mobile station by judging a propagation environment of a forward link and backward link based on a CRC(Cyclic Redundancy check) in a mobile station of a mobile communication system which adapts a CDMA method and dynamically performing a backward closed loop power control.
To achieve the above object, there is provided a backward closed loop power control apparatus for a mobile communication system which comprises a demodulation and PCB extraction unit for receiving a communication frame transmitted from a base station, demodulating the received communication frame and extracting a PCB having an allocated transmission power up or down instruction information, a CRC checking unit for checking a CRC with respect to the communication frame transmitted from the base station, a control unit for outputting a transmission power up or down control signal in accordance with a PCB extracted by the demodulation and PCB extraction unit and a CRC result value checked by the CRC checking unit, and a power adjusting unit for increasing the transmission power of the mobile station to the maximum value of the transmission power and decreasing the transmission power of the mobile station to the minimum value of the transmission power in accordance with a transmission power up or down control signal outputted from the controller.
To achieve the above objects, there is provided a backward closed loop power control method for a mobile communication system in a backward closed loop power control method of a mobile station in a CDMA mobile communication system for controlling a transmission power of a mobile station in accordance with a transmission power up or down instruction of a base station, which method comprises a communication frame receiving step in which the mobile station receives a communication frame including a PCB having an allocated transmission power up or down instruction information, a PCB extraction step for demodulating a communication frame received in the communication frame receiving step and extracting the PCB, a CRC checking step for checking the CRC with respect to the communication frame received in the communication frame receiving step after the PCB extraction step, and a transmission power control step for increasing or decreasing the transmission power of the mobile station by a previously set value based on the value of the PCB extracted in the PCB extraction step and the CRC result value checked in the CRC checking step.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.